And about the OTHER boss (bc I just thought about this)...
Sans is the exact opposite of Undyne.
Undyne is the chief of the royal guard, incredibly strong and hard working, and has powerful hope that monsterkind will one day reach the surface once more.
Sans runs a hotdog stand, is the weakest monster in the underground, extremely lazy, and gave up hope of going "home" long ago.
Yet they both fight, for the same reason. So that you might give up, and leave their world.